With the growth of technology related to electronic devices, there is an increasing demand for a small, thin, lightweight and high-capacity battery in the market. Also, coping with the development of electric vehicles or fuel cell vehicles, a battery adapted to a vehicle is required.
A lithium-based secondary battery which is one of rechargeable batteries is being widely used for a great variety of portable electronic devices such as a mobile phone, a notebook computer, a small video camera, and the like. Normally, a lithium secondary battery is composed of a cathode (positive electrode), an anode (negative electrode), and an electrolyte. In the lithium secondary battery, lithium ions move from a cathode active material to an anode active material during charge, and the reverse process occurs during discharge. While moving between the electrodes during charge and discharge, lithium ions transfer electrical energy.
A typical lithium secondary battery has, however, some problems such as safety lowering due to overheating, a low energy density of about 360 Wh/kg, and a low power output.
In order to solve such problems of a typical lithium secondary battery, a lithium-sulfur secondary battery capable of realizing a high energy density and a high power output are being developed. Because of using sulfur, as a cathode active material, having a high capacity per mass, the lithium-sulfur secondary battery can realize a high energy density.
The lithium-sulfur secondary battery has, however, a problem that polysulfide which is an intermediate product of lithium and sulfur flows into the electrolyte during subsequent charge/discharge reactions. This causes a capacity reduction due to a continuous loss of a reversible sulfur active material, a self-discharge due to a shuttle reaction of the polysulfide in the electrolyte, and a formation of a high resistance film at extraction on an anode surface, thereby deteriorating the long-term lifetime properties of the lithium-sulfur secondary battery.
As a solution of these problems, many techniques for inserting sulfur into a porous carbon nanostructure having a high electrical conductivity and a large internal space have been developed. These techniques are to restrict a movement of the polysulfide produced during the charge/discharge reactions within the structure, thereby minimizing the occurrence of the above problems and improving the long-term lifetime properties of the lithium-sulfur secondary battery.
However, the porous carbon nanostructure has difficulty in increasing an inner pore, so that there is a limit to increase the content of sulfur inserted into the porous carbon nanostructure. As a result, even though having an advantage of improving the long-term lifetime properties of the lithium-sulfur secondary battery, the carbon nanostructure technology is disadvantageous in energy density as compared with a lithium-sulfur secondary battery including a cathode using sulfur itself as an active material.